Using custom hash function with HAProxy LoadBalancer

Question

I have configured a load balancer using HAProxy on frontend and on the backend I have 4 server serving the request using Apache web server.

#HAProxy for web servers
frontend web-frontend
  bind IPADDRESS_LB:80
  mode http
  default_backend web-backend

backend web-backend
  balance roundrobin
  server web-server1 IPADDRESS1:80 check
  server web-server2 IPADDRESS2:80 check
  server web-server3 IPADDRESS3:80 check
  server web-server4 IPADDRESS4:80 check

My requests are served by either of the machines in a round robin mechanism. But now I want to implement my own algorithm to send the request based on the request parameters, eg IP.

Like Implementing my own hash function based on the result of which I can route my request to either of the backend servers.

How can I do that?

Answer 1

You can parametrize what to use in the hash (source with examples : HaProxy's blog)

In your case, simply use balance source to hash based on the source (IP) :

#HAProxy for web servers
frontend web-frontend
  bind IPADDRESS_LB:80
  mode http
  default_backend web-backend

backend web-backend
  balance source
  hash-type consistent
  server web-server1 IPADDRESS1:80 check
  server web-server2 IPADDRESS2:80 check
  server web-server3 IPADDRESS3:80 check
  server web-server4 IPADDRESS4:80 check

(hash-type consistent makes sure that distinct IP hashes give the same result)

Or based on the URL parameter named ip :

#HAProxy for web servers
frontend web-frontend
  bind IPADDRESS_LB:80
  mode http
  default_backend web-backend

backend web-backend
  balance url_param ip
  hash-type consistent
  server web-server1 IPADDRESS1:80 check
  server web-server2 IPADDRESS2:80 check
  server web-server3 IPADDRESS3:80 check
  server web-server4 IPADDRESS4:80 check

To base the hash on multiple parameters, you can :

1. add a header to the request containing the data you want a hash of
2. use this header for balancing.

Example from HaProxy's blog :

  backend bk_static
    http-request set-header X-LB %[req.hdr(Host),lower]%[req.uri,lower]
    balance hdr(X-LB)
    hash-type consistent